The present invention relates to methods and arrangements in a radio access network of a cellular mobile network. An example of such a radio access network is the UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System 100 connected to the Core Network (CN) 200. The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS 100 comprises at least one Radio Network Controller 110. One RNC may be connected to another RNC via the Iur interface 160. Furthermore, the respective RNC 110 controls a plurality of Node Bs 120, 130 also referred to as radio base stations. The Node Bs are connected to the RNC by means of the Iub interface 140. Each Node B covers one or more cells and is arranged to serve the User Equipment (UE) 300 within said cell. Finally, the UE 300, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface 150. The network of FIG. 1 is also referred to as a WCDMA network and is based on the WCDMA standard specified by the 3:rd Generation Partnership Project (3GPP).
Requirements for mobile data access are increasing and demand for bandwidth is growing. To meet these needs the High Speed Data Packet Access (HSDPA) specification has been defined. HSDPA is based on WCDMA evolution standardized as part of 3GPP Release 5 WCDMA specifications. HSDPA is a packet-based data service in WCDMA downlink with data transmission peak rate up to 14.4 Mbps over a 5 MHz bandwidth. Thus HSDPA improves system capacity and increases user data rates in the downlink direction. The improved performance is based on adaptive modulation and coding, a fast scheduling function and fast retransmissions with soft combining and incremental redundancy. HSDPA utilizes a transport channel named the High Speed Downlink Shared Channel (HS-DSCH) that makes efficient use of valuable radio frequency resources and takes bursty packet data into account. This is a shared transport channel which means that resources, such as channelization codes, transmission power and infra structure hardware, is shared between several users.
In 3GPP Release 6, the WCDMA standard is further extended with the Enhanced Uplink concept, also denoted High Speed Uplink Packet Access (HSUPA), by introducing the Enhanced Dedicated Transport Channel, E-DCH. A further description can be found in 3GPP TS 25.309 “FDD Enhanced Uplink; Overall description”. This concept introduces considerably higher peak data-rates in the WCDMA uplink. Features introduced with E-DCH include fast scheduling, fast Hybrid Automatic Repeat reQuest (HARQ) with soft combining and Node B controlled scheduling. It should be noted that uplink is the direction from the UE to the node B.
Fast scheduling means that the Node B can indicate to each UE the rate the UE is allowed to transmit with. This can be done every TTI, i.e. fast. Thus, the network is able to control the interference in the system very well.
HARQ is a more advanced form of an ARQ retransmission scheme. In conventional ARQ schemes the receiver checks if a packet is received correctly. If it is not received correctly, the erroneous packet is discarded and a retransmission is requested. With HARQ the erroneous packet is not discarded. Instead the packet is kept and combined with a result of the retransmission. That implies that even if both the first transmission and the retransmission are erroneous, they may be combined to decode the packet correctly. This means that fewer retransmissions are required.
Enhanced uplink supports soft handover. FIG. 2 illustrates a scenario when a UE denoted SHO is in soft handover, i.e. the UE SHO is connected to more than one Node B simultaneously. Hence, Packet Data Units (PDUs) transmitted from the UE SHO can be received in several Node Bs. HARQ retransmissions are performed between the UE and the Node Bs in the active set. The active set is the set of Node Bs that the UE is simultaneously connected to. Each Node B that receives a PDU correctly forwards the PDU to the RNC. The RNC comprises means for reordering PDUs and means for selection combining, which implies that the better PDUs are selected and the other discarded. Consequently, PDU duplicates are filtered out and PDUs are delivered in-sequence to the network. Each Node B that is in the active set, but does not receive a PDU correctly, requests retransmissions of the PDUs by the UE. Thus, a consequence of soft-handover support is that multiple copies received in different Node B:s of the same PDU may be transmitted through the UTRAN transport network, i.e. between the RNC and The Node B on the Iub and between RNCs on the Iur interfaces.
In some cases, the Iub and/or Iur interface is the bottleneck in the UTRAN system and the Iub and/or Iur interface is limiting the performance. This can either be a permanent situation due to that the transport network is under-dimensioned, or it can occur during shorter time periods due to temporary overload situations on the Iub and/or Iur interface.
In case the Transport Network is the bottleneck, the transmission of duplicates in the Transport Network due to soft handover is highly undesirable. However, it is not desirable to remove the soft handover completely since this would give a performance loss over the radio interface. This is because soft-handover also implies power-control from multiple sites, and removing this aspect by reducing the active set to one cell/site may increase the inter-cell interference significantly. Thus, in a scenario when the Iub and/or Iur interface is limiting the performance, it would be desirable to retain soft-handover support in terms of power-control but avoiding that multiple copies of the same data units are transmitted through the Transport Network.